1. Field of the Invention
The present invention relates to radio transmitters and regulatory schemes for controlling their use and to methods and apparatus for automatically controlling a radio transmitter, for example, controlling the maximum electric field strength (i.e., in volts per meter) or power density which can be radiated from an antenna connected to the radio transmitter. The present invention also relates to a method and apparatus for automatically identifying an antenna connected to the radio transmitter.
2. Discussion of the Prior Art
Medical professionals and others have expressed concerns over health effects of exposure to Radio Frequency (RF) or microwave electromagnetic radiation. In response, governments and pseudo-governmental regulators around the world have promulgated increasingly complex and rigorous regulations in efforts to minimize such health effects.
Regulators and spectrum resource managers have also been confronted with an increasingly crowded electromagnetic spectrum because users of increasingly varied technologies incorporate wireless radio links into devices which were previously tethered by wires for passing data telemetry or the like.
Generally speaking, sovereigns are not inclined to simply adopt their neighbor's regulatory scheme, no matter how efficacious it may appear to be, and so the result is an ever-changing patchwork of regulatory schemes.
Special license-free bands have been set aside by spectrum management agencies around the world to permit users of low power wireless data telemetry radios to operate wireless links. For example, in the United States, the Federal Communications Commission (FCC) has designated license-free bandwidth segments of the radio frequency spectrum and made them available for industrial, scientific and medical (ISM) uses. In order to minimize problems with electro-magnetic compatibility (EMC) between un-licensed radio transmitters and other radio systems, even more complex and rigorous regulations have been promulgated to control radiation of RF or microwave energy.
To cite a concrete example, referring to the Oct. 1, 1997 edition of Title 47 of the Code of Federal Regulations (47 C.F.R.), U.S. telecommunications regulations, such as 47 C. F. R. §15.245, §15.247 and others, limit maximum peak output power and electric field strength, as measured in units of volts (or millivolts) per meter. Other provisions require that, for certain kinds of installations, if transmitting antennas of directional gain greater than 6 dBi are used, the peak output power from the transmitter or intentional radiator shall be reduced to below set values. For example, systems operating in the 2400–2483.5 MHZ band that are used exclusively for fixed, point-to-point operations may employ transmitting antennas with directional gain greater than 6 dBi, provided the maximum peak output power of the intentional radiator is reduced by 1 dB for every 3 dB that the directional gain of the antenna exceeds 6 dBi.
Section 15.249 provides that transmission within the ISM bands, 902–928 MHZ, 2400–2483.5 MHZ and 5725–5875 MHZ shall be limited in electric field strength to 50 millivolts per meter at the fundamental frequency, and at 24.0–24.25 GHz shall be limited to 250 millivolts per meter at the fundamental frequency. Field strength limits are specified at a distance of 3 meters and the field strength of harmonics must be less than or equal to one hundredth the limit set for the fundamental frequency (e.g., at 902 MHZ, 500 microvolts per meter for harmonics).
A number of modern wireless data transmission systems can be configured for use with more than one antenna. Section 15.203 requires that a “unique coupling” be used to “ensure that no antenna other than that furnished by the responsible party shall be used with the device.” Apart from “professional installations”, the only alternative compatible with the techniques of the prior art under this regulatory scheme is use of a permanently attached antenna. The regulatory scheme therefore requires a wireless data transmission system vendor to try to anticipate the mischief of the user.
If antennas having a variety of efficiencies or directivities are to be employed by a user, then observing the maximum field strength or peak power limitations set forth by the regulations in the various jurisdictions can become a cumbersome problem requiring time consuming adjustment of transmitter output power for each application. At present, the most straightforward way to do this is to have a competent RF technician calculate the maximum field strength for a given antenna/transmitter combination and then de-power or attenuate the transmitter RF power output for a given antenna's gain. This process makes it difficult and expensive to design and field wireless data transmission systems.
There is a need, therefore, for a method to comply with the world-wide patchwork of regulatory schemes controlling RF and microwave radiation; preferably, the method does not require the wireless data transmission system vendor to send along that increasingly hard to find competent RF technician.